1. Field of the Invention
The present invention relates to a display control technique for a projection type display apparatus.
2. Description of the Related Art
A variety of high-quality video sources including video sources such as high-definition television (HDTV) broadcasting are available. In addition, presentations and the like in meeting rooms are generally conducted with videos on computers. For this reason, increasing demands have arisen for improvement in the image quality of video apparatuses and increases in the screen sizes of the apparatuses used for such purposes. A rear-projection type display apparatus is available as a promising technique of implementing such a high-quality, large-screen display apparatus.
An apparatus designed to project videos by using a high-luminance CRT (Cathode-Ray Tube) has initially been the mainstream of rear-projection type video apparatuses. Recently, however, a rear-projection type video apparatus using a light valve such as a transmissive liquid crystal, a reflective liquid crystal or a digital micro-mirror device (DMD) has become the mainstream. Such an apparatus is designed to apply light to the light valve and enlarge/project video light from an image surface of the light valve onto the screen by using the projection optical system. Many such apparatuses use, as light valves, LCOS (Liquid Crystal On Silicon) panels having characteristics that inter-pixel joints are inconspicuous and the optical control efficiency is high. Note that an LCOS panel is a reflective liquid crystal panel obtained by forming a liquid crystal layer on a semiconductor substrate. This apparatus uses, as a light source, a discharge lamp such as a superhigh-pressure mercury lamp or a metal halide lamp owing to its high luminous efficacy.
Note that a projector designed to project video light from the front surface of the screen is called a front projector, and a projector designed to project video light from the rear surface of the screen is called a rear projector. In this case, the “front surface” indicates the side where the observer is located, and the “rear surface” indicates the side opposite thereto.
Various kinds of discharge lamps used in these projection type display apparatuses have the following characteristics. A change in the temperature of a discharge lamp affects the state of mercury or the like in the lamp, resulting in a change in internal pressure. This will cause changes in luminance and white balance. In order to solve this problem, for example, Japanese Patent No. 3299058 discloses an arrangement which corrects a color temperature change caused by a rise in the temperature of a metal halide lamp in long-term continuous use, on the basis of the temperature detected by a temperature detection means provided near the lamp. In addition, Japanese Patent Laid-Open No. 05-173107 discloses a technique of measuring the time of lighting of a lamp and correcting white balance in accordance with the time of lighting. Furthermore, Japanese Patent Laid-Open No. 05-232428 discloses a technique of directly detecting the balance between the respective colors, R, G, and B, by providing a light-receiving unit which receives illumination light, and detecting the variation amount of an emission spectrum distribution. This reference discloses an arrangement which corrects a temporal change in the color reproducibility of a video upon a change in emission spectrum distribution based on a total emission time.
Particularly from the start of lighting until a predetermined period of time has elapsed, the rate of change in the amount of mercury vapor inside is very high. Therefore, variations in the color of the discharge lamp are very large. FIG. 10 is a graph based on an actual measurement showing a temporal change in color difference ΔE at the start of lighting of the discharge lamp. Referring to FIG. 10, the color difference ΔE from illumination light at a rise time is plotted along the elapsed time with reference to a white spot of illumination light in a long-term continuous state (stable state). As is obvious from FIG. 10, the color difference ΔE changes greatly in a period of several minutes from the start of lighting, and the rate of change is high. Referring to FIG. 11, a change in the position of a white spot on an x-y plane in a Yxy coordinate system is plotted along the elapsed time. Obviously, the position of a white spot greatly changes along with an elapsed time t in the direction indicated by the arrow on the x-y plane. That is, it is obvious that the white balance greatly changes within the measurement period in FIG. 11.
The measurement speed of conventional temperature measurement near a lamp is too low to follow such a short-term change (fast change), resulting in an incapability of properly correcting the change. In addition, performing only time measurement leads to a high possibility of occurrence of errors in correction values due to the influences of variations in lamp bodies, ambient temperature, and the like. Furthermore, installing a color measurement sensor capable of handling fast color changes will cause an increase in cost and limit the degree of freedom in terms of the internal arrangement of the apparatus.